1. Field of the Invention
The present invention relates to a light emitting diode circuit; in particular, to a compensation method and a compensation apparatus for a light emitting diode circuit having the light emitting diode.
2. Description of Related Art
As development of the semiconductor industry, light emitting diodes (LEDs) are not only widely utilized for lighting to replace traditional light bulbs which has low energy efficiency but also applied to the field of display. After thin film transistor liquid crystal displays (TFT-LCDs) with backlight module virtually replace cathode ray tube (CRT) displays organic-LED (OLED)-based display devices were further introduced as they are associated with characteristics of flexibility, higher efficiency, higher image contrast, and faster response time, all of which may propel the OLED-based display devices to be one mainstream displays in the future.
Generally, an active-matrix organic-LED is driven by a driving circuit with two transistors and one capacitor (2T1C). Please refer to FIG. 1. FIG. 1 shows a circuit diagram of a traditional organic-LED circuit. The traditional organic-LED circuit 1 comprises a first transistor T1, a second transistor T2, and a capacitor CS. A first end of the first transistor T1 is coupled to a high voltage level VDD. A second end of the first transistor T1 is coupled to an input end of the organic-LED OLED. An output end of the organic-LED OLED is coupled to a grounding end. A second end of the second transistor T2 is coupled to a control end of the first transistor T1. A first end of the second transistor T2 receives a pixel signal DATA. The control end of the second transistor T2 receives a scanning signal SCAN. The capacitor CS has a first end coupled to the control end of the first transistor T1 and the second end of the second transistor T2. A second end of the capacitor CS is coupled to the grounding end.
The organic-LED OLED emits light according to an output current IOLED generated by a driving voltage VDATA of the first transistor T1. The magnitude of the light is controlled by the pixel signal DATA and the scanning signal SCAN. After the first transistor T1 has been in operation for an extended period of time, stress of the gate to source voltage Vgs (or increase in voltage level of the same) would cause a threshold voltage Vth of the first transistor T1 to drift to a higher voltage level. Therefore, the output current IOLED and the output voltage VOLED of the organic-LED would decrease, negatively affecting light-emitting efficiency of the OLED and reducing lifespan thereof.
FIGS. 2A and 2B show curves of simulated output current of the traditional organic-LED circuit with respect to the threshold voltage of the first transistor. FIG. 2A is represented in terms of a micro-ampere (μA) with the output current in FIG. 2B being normalized. As shown in FIG. 2A and FIG. 2B, if the threshold voltage Vth of the first transistor T1 increases from 0V to 3V, the output current IOLED of the organic-LED OLED may decrease from 2.05 micro-amperes to 1.3 micro-amperes.
Since the output current of the organic-LED would decrease to undermine the light-emitting efficiency of the organic-LED, the quality of lighting instruments or display devices which utilize the traditional LED circuits deteriorate or degrade as the result.